Process for wood acetylation and product thereof

ABSTRACT

A process for the acetylation of wood comprising submerging the wood in an acetylation fluid under pressure, and subsequently heating the wood under controlled conditions to initiate two distinct exothermic reactions. The process permits the simultaneous upgrading of large quantities of commercial wood sizes having a natural durability class 4 or class 5 to a unique product of durability class 1 or class 2.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 12/865,571filed on Sep. 27, 2010 now pending, entitled PROCESS FOR WOODACETYLATION AND PRODUCT THEREOF, which is the National Stage ofInternational PCT/GB2009/000268, filed on Jan. 30, 2009 which claims thebenefit of Foreign Patent Application GB0801880.6, filed on Feb. 1,2008, Foreign Patent Application GB0814785.2, filed on Aug. 13, 2008,and Foreign Patent Application GB0823012.0, filed on Dec. 18, 2008, thedisclosure of which is incorporated by reference herein.

The present invention relates to the modification of components of wood,and in particular the modification of solid, non-durable wood species byacetylation, to improve desirable characteristics such as durability,dimensional stability, stability to ultraviolet light and thermalconductivity. Non-durable wood species are generally considered to bethe soft wood derived from coniferous trees, and non-durable hardwoodssuch as described in BRE durability classes 4 and 5 (see below).

The benefits of acetylating solid soft woods, solid non-durablehardwoods and wood veneers (hereinafter referred collectively to wood)have been extensively studied on a laboratory scale and documented inacademic and trade publications. Broadly defined, wood acetylationresults in the conversion of hydroxyl groups in wood components toacetyl groups. This chemical modification thus has the effect ofconverting hydrophilic hydroxyl groups to hydrophobic acetyl groups.

Prior art documentation has mostly focused on improvements in thedurability and dimensional stability of small pieces of wood primarilyfor laboratory investigations and are of little commercial significance.Durability is essentially the resistance of wood to natural rottingprocesses, such as those brought on by fungi, while dimensionalstability can be described as a reduction in swelling and shrinkage whenwood is subjected to cycles of wetting by water, or by humidity,followed by drying.

In early wood acetylation work (Forest Products Journal, February 1964,page 6, Goldstein, Dreher and Cramer), acetic anhydride was dissolved indiluents, such as xylene and toluene, to assist in the penetration ofponderosa pine, sugar maple and white oak. The applicability of thistechnique to commercially used sizes of wood, where cross-grainpenetration is essential in terms of durability and dimensionalstability, is not addressed. Moreover, the by-product streams weremixtures of difficult to separate blends of acetic anhydride, aceticacid and xylene.

The processes described in European patent 213 252 are principallydirected to the acetylation of wood fibres and chips, and not tocommercial sizes of wood. The examples given for solid wood are of theapplication of the processes on a laboratory scale to very small pieces.

European patent 680810 attempts to address the acetylation of solid woodof commercial dimensions, but in doing so relies on an impregnation byacetic anhydride in the absence of effective control over the reactionheat that is generated in the wood. Temperatures in the wood cantherefore rise to levels which can cause internal cracking and charring,significantly reducing the strength of the acetylated wood. EP 680810also relies on the characterising step of post treating the acetylatedwood with steam. The applicants have found this to be largelyineffective in removing, residual acetic acid.

International patent publication WO 2005/077626 is concerned almostexclusively with the acetylation of oven-dried wood, meaning it is freeof all but traces of moisture. While this process may be applicable tothe short lengths of wood used in the reported experiments, oven-driedwood is essentially unknown in the commercial world, primarily due tothe damage caused by severe drying of commercial lengths, e.g. 2 to 4metres. The application of super-heated acetylation fluid to commercialwood sizes, typically containing 12 to 20% moisture, results in a rapidgeneration of heat further increasing the high temperature of theacetylation fluid and driving the overall temperature inside the wood tolevels which can cause cracking and charring of the internal, structureof the wood.

A specific shortcoming of the prior art generally is so-called ‘envelopetreatment’. This is illustrated in “Acceptance Criteria for AcetylatedWood Preservative Systems”—Document No AC297 published by ICC EvaluationServices 1 March 2005. Envelope treatment is the acetylation of woodnear its surface, leaving the interior wood at an ineffective level ofacetylation or with no acetylation at all. Wood acetylated to anineffective depth can be exposed directly to moisture when theacetylated piece is planed, profiled or cross-cut. Acetylated woodallows moisture to pass into and out of the entire cross section. In theproperly acetylated parts, moisture encounters protected cell walls andcannot be used to support fungal growth. In poorly acetylated orunacetylated parts, moisture adheres to the cell walls where it assistswood-rotting fungi. The net result of moisture reaching such parts isthat the wood rots from the interior outwards.

Other common wood preservation treatments involve the use of chromatedcopper arsenate (CCA), quaternary copper salts, pentachlorophenol, andcreosote, but unlike acetylation, these treatments are carried out atambient temperature and do not involve exothermic reactions.

Publications on wood acetylation generally deal with the fundamentalchemistry and the properties of the wood obtained: These publicationsprovide little guidance on how to achieve volumetric efficiency in thewood acetylation reaction, little guidance on how to initiate thereaction except by the crude application of heat, little guidance on howto dissipate the heat generated by exothermic reactions and no guidanceon how to profile the temperature around a multiplicity of wood piecesstacked in a reactor so that uniform acetylation is obtained in everypiece and throughout every piece and for every piece in the reactor.

Much of the work appearing in the literature and in earlier patentdocuments is relevant solely to the durability and dimensional stabilityof small, laboratory prepared samples of wood. These studies areessentially silent on the incorporation, or the achievement, of thesedesirable properties when larger wood sizes, such as would be commonlyused in the building industry and trades, are acetylated.

Thus, the technical problem to be solved is the uniform penetration ofwood pieces of commercial sizes by acetic anhydride using only thepenetration achieved perpendicular to the grain and the management ofreaction heat. Failure to manage heat within the wood, especially at thecore will, at minimum, result in strength reduction(s) in the acetylatedwood or in partial acetylation. At worst, it will result in the outrightcharring of the wood due to unreleased reaction heat. Not only must heatmanagement be applied uniformly throughout the cross-section of eachwood piece, it must be applied uniformly to the entire length of eachpiece and equally to each piece in a reactor.

Wood durability is achieved by protecting the cell wall from fungalgrowth, which, in turn, requires the conversion of hydroxyl groups toacetyl groups. This must be done in a uniform fashion across the entirewood piece.

Dimensional stability is achieved by swelling the kiln-dried wood backto its green volume without exceeding the elastic limits of the cellwall. Again, this must be done uniformly across the entire wood piece.

Thus, the present invention provides a process for the acetylation ofwood comprising the steps:

-   -   (a) in a reaction pressure vessel submerging wood having a        moisture content of 6% to 20% by weight in an acetylation fluid        at a temperature of 10° C. to 120° C.    -   (b) increasing the pressure in the vessel to 2 to 20 bar for a        period of 10 to 300 minutes    -   (c) removing excess acetylation fluid from the vessel    -   (d) introducing into the vessel an inert fluid, circulating and        heating the fluid until the internal temperature of the wood        begins to show an exotherm, controlling the supply of heat to        the wood until the exotherm is complete and maintaining the        internal temperature of the wood below 170° C.    -   (e) heating the circulating fluid to a temperature of 85° C. to        150° C. for a time of 10 to 30 minutes to initiate a second        exothermic reaction, controlling the supply of heat to the wood        until the exotherm is complete and maintaining the internal        temperature of the wood below 170° C.    -   (f) removing the circulating fluid and allowing the acetylated        wood to cool to ambient temperature.

In (a) the moisture content of the wood is preferably less than 12% byweight, and desirably less than 8% by weight. The acetylation fluid maycomprise 60% to 95% by volume of acetic anhydride and 5% to 40% byvolume acetic acid. Preferably, the fluid comprises 80% to 92% by volumeacetic anhydride and 8% to 20% by volume acetic acid. Desirably, theacetylation fluid is at a temperature of 35° C. to 55° C.

In (b) the pressure in the vessel is preferably increased to 10 to 15bar for 30 to 90 minutes, depending on the permeability and dimensionsof the wood to be treated. Pressurisation with nitrogen is preferred butother inert gases, such as carbon dioxide, may also be used.

In (c) by excess is meant acetylation fluid which has not impregnatedthe wood. It may be removed from the pressure vessel by using existingpressure within the vessel, eg nitrogen gas, to force the fluid into astorage vessel, or by pumping the fluid out while maintaining a nitrogenpressure in the vessel.

In (d) the inert fluid (i.e. a fluid which does not react with aceticanhydride or acetic acid) is typically gaseous nitrogen, gaseous carbondioxide or flue gas, which is heated to a temperature of from 20° C. to120° C. The commencement, duration and completion of the exotherm isdetected and monitored by thermocouples located in the wood. In someinstances, the gaseous fluid, e.g. nitrogen, may be partially or fullysaturated with non-inert acetic anhydride and/or acetic acid. This canrange from 20% to full saturation (100%).

In (d) and (e) it may be necessary to cool the circulating fluid toavoid the internal temperature of the wood (detected by thermocouples)exceeding 170° C., preferably not exceeding 155° C.

In (e) the preferred circulating fluid temperature is 100° C. to 135°C., and the preferred time 10 to 15 minutes.

During the cooling of the acetylated wood residual acetic anhydride andacetic acid by-product may be removed, for example, by evaporation undervacuum.

In some process circumstances it is preferred to reduce the moisturecontent of the wood to be acetylated, by first introducing the wood intothe pressure vessel and reducing the pressure therein to, for example,0.05 to 0.5 bar for 10 to 300 minutes, preferably 30 to 120 minutes,depending upon the permeability of the wood, before submerging the“dried” wood in an acetylation fluid. The vacuum is convenientlyreleased by allowing the acetylation fluid to enter the reaction vessel.

The present invention is of particular value in the acetylation ofcommercial sized wood pieces, resulting in acetylated wood with uniformand predictable properties. The invention is especially applicable towood pieces to be acetylated to at least 14% by weight acetyl at theirgeometrical centre which have a width of from 2 cms to 30 cms, athickness of from 2 cms to 16 cms, and a length of from 1.5 metres to6.0 metres. Preferably, the wood pieces have a width of from 2 cms to 10cms, a thickness of from 2 cms to 10 cms, and a length of from 1.5metres to 4.0 metres.

It is an important characteristic of the acetylated wood of the presentinvention that it retains essentially all its original strength andappearance. It is common for wood acetylation processes described in theprior art to result in treated wood having a darkened or discolouredsurface, which can significantly detract from the aesthetic appearanceof the product. In the present invention, such a result is a rareoccurrence, and should it occur, can readily be removed by planning,sanding or profiling. Other benefits that have been recognised aresuperior wet stiffness, dimensional stability and machine workability.

In cases where the wood to be acetylated has a high moisture content, alow permeability, or a high density, it may be necessary for a secondimpregnation with an acetylation fluid followed by a second acetylation,to achieve the desired acetyl levels (steps (a) to (f)—page 4). In suchcases, partially acetylated wood according to the present invention,still partially wet with acetic anhydride and acetic acid, has beenfound to take up more acetylation fluid than expected, and can do sowithout the action of hydrocarbon diluents to assist in penetration oracting as a carrier fluid.

The present invention also provides acetylated wood having unique, i.e.not hitherto known or attainable, wet stiffness (modulus of elasticity)and wet strength (modular of rupture). In the case of radiata pine,unacetylated samples were found to have a dry stiffness of approximately10540 N/mm² and a wet stiffness of the order of 6760 N/mm², i.e. a lossof 36% in dry stiffness, whereas for the same pine after acetylation thecorresponding results for stiffness were 10602 N/mm² and 9690 N/mm², aloss of stiffness of less than 10%, namely 8.6% (reference BS EN408:2006—British Standards Institute—BSI). Also, the acetylated wood canpossess significantly improved dimensional stability in terms of radialand tangential shrinkage, when compared with unacetylated wood (methodsfor the measurement of radial and tangential shrinkage in wood are welldocumented). See Table 1 below, where very little shrinkage has occurredin the acetylated samples.

TABLE 1 Wood Sample Radial Shrinkage* $\frac{R_{2}}{R_{1}}$ TangentialShrinkage* $\frac{T_{2}}{T_{1}}$ Radiata Pine unacetylated R₁ 1.2 0.33T₁ 2.2 0.32 acetylated 20% R₂ 0.4 T₂ 0.7 Scots Pine unacetylated R₁ 1.00.30 T₁ 2.4 0.29 acetylated 20% R₂ 0.3 T₂ 0.7 Beech unacetylated R₁ 1.20.58 T₁ 2.5 0.44 acetylated 20% R₂ 0.7 T₂ 1.1 *at 60% to 90% relativehumidity

Preferably, acetylated wood pieces have a starting width of 2 cms to 30cms, a thickness of from 2 cms to 16 cms, and a length of from 1.5metres to 6.0 metres. Desirably, the wood pieces have a starting widthof from 2 cms to 10 cms, a thickness of 2 cms to 10 cms, and a length offrom 1.5 metres to 4.0 metres.

The present invention also offers the unique prospect of thesimultaneous upgrading of large quantities of commercial wood sizeshaving a natural durability class 4 or class 5 to durability class 1 orclass 2. Reference the widely recognised “five-level” wood speciesdurability grading devised by the Building Research Establishment Ltd,Garston, United Kingdom, and published in BRE Digest 296,1985 (replacedby Digest 429, 1998):

Class 1 is termed as “very durable”, i.e. 25 years, or more, ofin-ground contact with minimal loss of strength or mass. Example: teak,robinia

Class 2 is termed as “durable”, i.e. greater than 15 years but less than25 years of in-ground contact with minimal loss in strength or mass.Example: American white oak, western red cedar.

Class 3 is termed as “moderately durable”, i.e. greater than 10 yearsbut less than 15 years of in-ground contact with minimal loss ofstrength or mass. Example: European larch, sapele

Class 4 is termed as “non-durable”, i.e. greater than 5 years but lessthan 10 years of in-ground contact with minimal loss of strength ormass. Example: radiata pine, yellow pine, Douglas fir.

Class 5 is termed as “perishable”, i.e. up to 5 years of in-groundcontact with minimal loss of strength or mass. Example: poplar, Europeanbeech.

By improving the durability of plantation grown soft woods theacetylated wood of the present invention can be substituted for tropicalwood, and for wood treated with toxic chemicals such as arsenic, copper,chromium and pentachlorophenol. In addition to the commercialadvantages, the benefits t6 the environment are self-evident, namely, areduction in the use of native tropical hard woods and the avoidance oftreatments employing toxic chemicals.

A significant advantage of the present invention is the drying of theacetylated wood using the same circulating inert fluid used in theacetylation reaction. The fluid (when gaseous) is passed throughcondensers where a mixture of acetic acid by product and residual aceticanhydride may be removed.

Another significant advantage of the current invention is that theliquid mixture condensed from the circulating inert fluid is free fromwater, high-boiling point wood extracts and debris, thus avoiding theneed for an expensive recovery process.

At least two options are available for recycling the recovered liquidmixture. One is to flash distil the liquid to effect a crude separationof the acetic acid from the anhydride. The recovered acetic acid maythen be fed to a ketene cracker, and the recovered acetic anhydriderecycled to the wood acetylation process.

A second option is to distil acetic acid from the recovered liquid, andsell it as technical grade acid. The unused acetic anhydride collectedin the condensate is then returned to the wood acetylation process.

The present invention uses high-pressure liquid chromatography (HPLC) toquantify the acetate ion concentration resulting from the saponificationof the acetyl groups. This gives a direct measure of acetyl contentrather than general weight gain. And, it can be applied to small areasof each acetylated piece. In addition, calibrated Fourier transforminfrared spectrophotometers (FTIR) and near infrared spectrophotometers(FTNIR) can be used to measure the acetyl content of slices of wood thatare 2 mm in thickness and 4 mm×2 mm in area. This allows confirmation ofacetylation on pinpoint sized spots and allows the gradient ofacetylation across individual pieces to be viewed.

The following examples are intended only to illustrate the presentinvention. They are not to be taken as limiting the invention in anyway.

EXAMPLE 1

About 0.4 M³ of rough-sawn, radiata pine boards from Chile, with 11%moisture, were separated with 15 mm stickers vertically andhorizontally. The boards were 3.9 meters long×55 mm thick by 130 mm wideand composed of largely sapwood with some heartwood. The wood was loadedinto a 2.5 cubic meter liquid capacity reaction pressure vessel. Thevessel was equipped with a gas circulation loop.

Optionally, a vacuum was applied to the boards. In the present examplethe vacuum was 0.1 to 0.2 bar absolute, and was applied for 30 minutes.Acetylation fluid (90% acetic anhydride and 10% acetic acid at ambienttemperature) was used to release the vacuum and submerge each board.Nitrogen was used to increase the pressure to 10 bar absolute and heldfor 90 minutes. The free liquid was drained leaving the saturated boardscontaining 1.5 to 1.7 kilograms of liquid per kilogram of wood.

Nitrogen gas, saturated with acetic anhydride, was used as the heatingmedia for the acetylation reaction. The purpose of saturating the gaswith acetic anhydride is to avoid evaporation of acetylation fluid atthe wood's surface before and during acetylation. The circulating loophad a volume of four cubic meters, which when combined with the volumeof the reactor, provided approximately six cubic meters of gas for 0.4M3 of wood. During the acetylation cycle, the pressure in gascirculation loop varied between 1.1 to 1.9 bar absolute.

When the temperature of the circulating gas reached approximately 60°C., the reaction between the acetic anhydride and the moisture in thewood, and the acetylation of the wood, began. This was evidenced by anincreasingly rapid rise in temperature measured by thermocouplesinserted into several of the boards. Heat generated by this reactionplus some additional heating by the circulating nitrogen increased thetemperature inside the wood to 130° C. to 140° C., where the reactionbetween wood hydroxyl groups and the acetic anhydride began.

During the acetylation period of approximately 90 minutes, it wasnecessary to condense some of the acetic acid-acetic anhydride vapour tocontrol the pressure and temperature of the circulating gas. Therecovered liquid had a composition of approximately 5% acetic anhydrideand 95% acetic acid.

At the end of the acetylation period, more of the unused aceticanhydride and the by-product acetic acid were condensed from thecirculating gas. The temperature was increased gradually to about 130°C. These actions combined to drive volatile materials to the surface ofthe boards and evaporate them into the gas stream. The recovered liquidwas totally free of water and was composed of 3% to 4% acetic anhydrideand 96% to 97% acetic acid.

Finally, the boards are dried to a point where they contained about 15grams to 30 grams of acetic acid per kilogram of wood. Any surfaceimperfections were removed by planing, sanding or profiling.

The unused acetic anhydride was available for recovery because no waterhad been added to the system.

The acetylated boards were found to have acetyl contents of 20 to 21% atthe surface and 18 to 20% at the core.

EXAMPLE 2

Approximately 0.4 M³ of rough sawn, southern yellow pine boards, whichcontain 12% moisture, were separated vertically and horizontally by 15mm stickers. The boards were 3.9 meters long×40 mm thick by 140 mm wideand composed of sapwood with some heartwood. The wood was loaded into a2.5 M³ liquid capacity reaction pressure vessel. The reaction vessel wasequipped with a gas circulation loop.

Optionally, a vacuum was applied to the boards. In the present examplethe vacuum was 0.1 to 0.2 bar absolute and was applied for 30 minutes.Acetylation fluid (92% acetic anhydride and 8% acetic acid at ambienttemperature) was used to release the vacuum and submerge each board.Nitrogen was used to increase the pressure to 10 bar absolute and heldthere for 60 minutes. The free liquid was drained leaving the saturatedboards containing 1.0 to 1.2 kilograms of liquid per kilogram of wood.

Nitrogen gas, saturated with acetic anhydride vapour, was used as theheating media for the acetylation reaction. The purpose of saturatingthe gas with acetic anhydride is to avoid evaporation of acetylationfluid at the surface of the wood before and during acetylation. Thecirculating loop had a volume of 4 M³, which when combined with thevolume of the reactor, provided approximately 6 M³ of gas for 0.4 M³ ofwood. During the acetylation cycle, the pressure in gas circulation loopvaried between 1.1 to 1.9 bar absolute.

When the temperature of the circulating gas reached approximately 80°C., the reaction between acetic anhydride and the moisture in the woodbegan. This was evidenced by an increasingly rapid rise in temperaturemeasured by thermocouples inserted into several of the boards. A secondexotherm from wood acetylation began at approximately 120° C. Duringthis first wood acetylation period, approximately 60 minutes, it wasnecessary to condense some of the acetic acid and acetic anhydridevapours to control the gas pressure and the temperature of thecirculating gas. The recovered liquid had a composition of approximately10% acetic anhydride and 90% acetic acid.

At the end of the first wood acetylation period, some of the unusedacetic anhydride and the by-product acetic acid were condensed from thecirculating gas. Vacuum was applied to the reactor to withdraw spentacetylation fluid from the wood. This fluid was pumped out of thereactor and vacuum applied a second time.

A fresh charge of acetylation fluid was used to relieve the vacuum,submerge each board and impregnate the wood for a second time. Thecomposition was 91% acetic anhydride, by weight, and the balance wasacetic acid. Nitrogen pressure was applied to approximately 10 barabsolute. After a pressure period of 60 minutes, the excess fluid waspumped from the reactor and the circulation of nitrogen saturated withacetic anhydride started at a pressure of about one bar absolute. Thetemperature of the saturated nitrogen was increased to about 90° C.

After the second wood acetylation was underway, as evidenced byincreasing pressure in the circulating gas and increasing temperaturesmeasured by thermocouples at the core of several boards, no additionalheat was applied to the circulating gas. As the pressure increased to1.5 to 1.8 bar absolute, fluid was condensed from the circulating gas toreduce pressure and/or reduce temperature. The composition of thecondensed fluid was 30% to 40% acetic anhydride with the balance beingacetic acid.

After the second wood acetylation period of approximately 60 minutes,the temperature of the circulating gas was gradually increased to 130°C. and liquid was condensed from a side stream of the circulating gas.

These actions combined to drive volatile materials to the surface of theboards and evaporate them into the gas stream.

The acetyl content of the boards was formed to vary from 20 to 22% atthe surface to approximately 15 to 17% at the core.

A further five examples are detailed in the following table:

Southern Radiata Yellow Mason Species Pine Pine Beech Poplar PineThickness, mm 100 38 25 40   38 Width, mm 150 145 120 140   145 Length,mm 4,000 3,000 2,000 4,000 3,000 No. Boards in reactor 616 2,287 4,6871,704 2,287 Horizontal space, mm 50 20 15 15   20 Vertical space, mm 1515 15 15   15 Density, kgs/m³ 470 505 540 430   540 Moisture content, wt% 10 7 8 7    9 Vacuum time, minutes at 0.2 90 45 60 35   90 bar(optional step) Weight of acetylation 120 to 125 120 to 125 124 to 128120 to 125 120 to 125 fluid delivered to reactor, MT Temperature ofacetylation 25 28 28 25   30 fluid at delivery time, ° C. Aceticanhydride 91 86 92 92   88 concentration in acetylation fluid, wt %Acetic acid 9 14 8 8   12 concentration in acetylation fluid, wt %Acetylation time, minutes 120 90 100 60   100 Acetylation pressure, bar11 15 12 12   10 Temperature in boards 50 to 60 60 to 65 60 to 65 55 to65 55 to 65 at initiation of water reaction, ° C. (first exotherm)Temperature in boards 120 to 130 115 to 135 120 to 130 115 to 135   120Acetyl content achieved, 20 to 22 17 to 20 17 to 19 16 to 18    14* wt %End use Window Decking Cladding Canal Decking frames Lining *a secondimpregnation, similar to the first, increased the acetyl content to20.5%.

In the two examples using 0.4 M³ of wood and in the five examples using37 to 40 M³ of wood acetylated wood was obtained which had Class 1durability as measured according to BS EN 350-1: 1994 (BSI).

In each of the seven examples the acetylated wood exhibited improvementsin dimensional stability of at least 70%, measured by anti-shrinkefficiency, when the samples were cycled between oven dry and 90%humidity.

In each of the seven examples, the UV stability of the acetylated woodshowed no measurable degradation in the 16-week acceierated laboratorytest described in BS EN 927-6 (BSI) or in one year exterior exposuretests described in BS EN 927-3:2000 (BSI).

In each of the seven examples, the thermal conductivity of theacetylated wood was reduced by approximately 40% when measured eitherparallel to the grain or perpendicular to the grain.

What is claimed is:
 1. Acetylated wood having: a) a radial shrinkageratio R₂/R_(i) of 0.27 to 0.64 wherein R₂ is the radial shrinkage afteracetylation and R₁ is the shrinkage before acetylation, and b) atangential shrinkage ratio T₂/T₁ of 0.26 to 0.48 wherein T₂ is thetangential shrinkage after acetylation and T₁ is the shrinkage beforeacetylation.
 2. The acetylated wood according to claim 1 wherein theratio R₂/R₁ is 0.30 to 0.58 and the ratio T₂/T₁is 0.29 to 0.44.
 3. Theacetylated wood according to claim 1 having a loss of dry stiffnessafter wetting of less than 10%.
 4. The acetylated wood according toclaim 3 wherein the loss of dry stiffness is not less than 8.6%.
 5. Theacetylated wood piece according to claim 1 having a width of 2 cms to 30cms a thickness of 2 cms to 16 cms and a length of from 1.5 metres to6.0 metres.
 6. The acetylated wood piece according to claim 5 having awidth of 2 cms to 10 cms a thickness of 2 cms to 10 cms and a length of1.5 metres to 4.0 metres.
 7. The acetylated wood according to claim 1acetylated to 14% to 22% by weight at its geometrical centre.
 8. Aprocess for the acetylation of wood comprising the steps: (a) in areaction pressure vessel submerging wood having a moisture content of 6%to 20% by weight in an acetylation fluid at a temperature of 10° C. to120°; (b) increasing the pressure in the vessel to 2 to 20 bar for aperiod of 10 minutes to 300 minutes; (c) removing excess acetylationfluid from the vessel; (d) introducing into the vessel an inert fluid,circulating and heating the fluid until the internal temperature of thewood begins to show an exotherm, controlling the supply of heat to thewood until the exotherm is complete and maintaining the internaltemperature of the wood below 170° C.; (e) heating the circulating fluidto a temperature of 85° C. to 150° C. for a time of 10 to 30 minutes toinitiate a second exothermic reaction, controlling the supply of heat tothe wood until the exotherm is complete and maintaining the internaltemperature of the wood below 170° C.; and (f) removing the circulatingfluid and allowing the acetylated wood to cool to ambient temperature,wherein the acetylated wood comprises: a) a radial shrinkage ratio R₂/R₁of 0.27 to 0.64 wherein R₂ is the radial shrinkage after acetylation andR₁ is the shrinkage before acetylation, and b) a tangential shrinkageratio T₂/T₁ of 0.26 to 0.48 wherein T₂ is the tangential shrinkage afteracetylation and T₁ is the shrinkage before acetylation.
 9. The processaccording to claim 8 wherein the moisture content of the wood is lessthan 12% by weight.
 10. The process according to claim 8 wherein in step(b) an inert gas is used for increasing the pressure in the vessel. 11.The process according to claim 10 wherein in step (c) the excessacetylation fluid is removed from the pressure vessel by pumping whilemaintaining the inert gas pressure in the vessel.
 12. The processaccording to claim 10 wherein in step (c) the excess acetylation fluidis removed from the pressure vessel by pressurising with the inert gas.13. The process according to claim 11 wherein the inert gas is nitrogen.14. The process according to claim 8, wherein the inert fluid in step(d) is selected from gaseous nitrogen, gaseous carbon dioxide or fluegas.
 15. The process according to claim 8, wherein the circulating inertfluid is cooled to avoid the internal temperature of the wood exceeding170° C.
 16. The process according to claim 8, wherein during the coolingof the acetylated wood residual acetic anhydride and acetic acidby-product is removed by evaporation under vacuum.
 17. The processaccording to claim 8 wherein the moisture content of the wood is reducedbefore submerging in acetylation fluid, by subjecting the wood to apressure of 0.05 to 0.5 bar for 10 to 300 minutes.
 18. The processaccording to claim 8, wherein the wood is acetylated to at least 14% byweight of acetyl at its geometrical centre.
 19. The process according toclaim 8, wherein there is a second impregnation with acetylation fluidfollowed by a second acetylation.
 20. The process according to claim 8wherein the acetylated wood is dried using the circulating inert fluid(when gaseous).
 21. The process according to claim 12 wherein the inertgas is nitrogen.